Liquefied gas unloading and deep evacuation system

ABSTRACT

A liquefied gas unloading and deep evacuation system may more quickly, more efficiently and more completely unload liquefied gases from transport tanks, such as rail cars, into stationary storage tanks or into truck tanks. The system may utilize a two stage compressor, an electric motor, a variable frequency drive, a four way valve, a three way valve, a two way valve, a programmable logic controller based control system and pressure and temperature transmitters. The valving enables deep evacuation of the transport or supply tank to more completely empty the transport tank. The programmable logic controller and variable speed drive may be used to variably control the speed of the two stage compressor so that the system may be running as fast as possible during changes in ambient temperature and/or different stages of offloading the liquefied gases without exceeding the compressor&#39;s horsepower limit.

BACKGROUND

Single stage compressors have been the traditional mechanism forliquefied gas transfer and vapor recovery applications for decades.Single stage compressors provide the highest capacity and efficiencyduring the liquid transfer process, as well as during the first portionof the vapor recovery process. These traditional means of liquidtransfer and vapor recovery, however, leave behind valuable product inthe supply tanks. The single stage compressor operation must stop due tolow volumetric efficiency or high discharge temperatures before all ofthe product can be transferred. Customers using these compressorstypically pay for the full contents of the supply tanks, usually a railcar or tank trucks, whether or not that customer is able to recover allof the product. Additionally, speed is an important factor in theoffloading of liquefied gasses. Increased offloading speeds can reducethe expenses of rail demurrage charges and labor. Traditional singlestage compressors may operate at a fixed operating speed. A fixedoperating speed does not allow the system to maintain a maximum fluidtransfer rate. Pressures and temperatures change throughout theoffloading process and throughout the different seasons of the year. Avariable compressor operating speed that adjusts based on the changingpressures and temperatures of the system, and thus is able to maintain amaximum fluid transfer rate throughout offloading, would be beneficial.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key factors oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

In one implementation a liquefied gas unloading and deep evacuationsystem may comprise a two stage compressor. The two stage compressor maycomprise a single stage mode and a two stage mode. The two stagecompressor may also comprise a first cylinder and a second cylinder. Thetwo stage compressor may be selectably changeable between the singlestage mode and the two stage mode at a predetermined pressure.

The liquefied gas unloading and deep evacuation system may also comprisea liquid trap fluidly coupled to the two stage compressor.

The liquefied gas unloading and deep evacuation system may also comprisea three way valve. The three way valve may fluidly couple the two stagecompressor with a four way valve. The three way valve may be selectablychanged between a first passageway and a second passageway. The firstpassageway may fluidly couple the first cylinder with the four wayvalve. The second passageway may fluidly couple the first cylinder withthe second cylinder of the two stage compressor.

The four way valve may comprise a first position and a second position.The first position may comprise a first passageway. The first passagewaymay fluidly couple the three way valve, the second cylinder, and asupply tank. The second passageway may fluidly couple a storage tankwith the liquid trap during the liquefied gas unloading.

The second position may comprise a third passageway, which may fluidlycouple the supply tank with the liquid trap. The second position mayalso comprise a fourth passageway, which may fluidly couple the threeway valve, the second cylinder of the two stage compressor, and thestorage tank during the single stage vapor recovery_mode.

The second position may also comprise the third passageway, which may befluidly coupled between the supply tank and the liquid trap. The secondposition may also comprise the fourth passageway, which may fluidlycouple the second cylinder of the two stage compressor, and the storagetank during the during the two stage mode.

To the accomplishment of the foregoing and related ends, the followingdescription and annexed drawings set forth certain illustrative aspectsand implementations. These are indicative of but a few of the variousways in which one or more aspects may be employed. Other aspects,advantages and novel features of the disclosure will become apparentfrom the following detailed description when considered in conjunctionwith the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

What is disclosed herein may take physical form in certain parts andarrangement of parts, and will be described in detail in thisspecification and illustrated in the accompanying drawings which form apart hereof and wherein:

FIG. 1 is a diagram of an example implementation showing a liquefied gasunloading and deep evacuation system during liquid transfer operation.

FIG. 2 is a diagram of an example implementation showing a liquefied gasunloading and deep evacuation system during vapor recovery operation.

FIG. 3A is a perspective view of an example implementation showing aliquefied gas unloading and deep evacuation system.

FIG. 3B is another perspective view of an example implementation showinga liquefied gas unloading and deep evacuation system.

FIG. 4 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system.

FIG. 4A is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system.

FIG. 5 is a schematic drawing illustrating the direction of fluid flowin an example implementation of a liquefied gas unloading and deepevacuation system.

FIG. 6 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system in single stagemode with the cylinders piped in parallel during the liquid transferphase.

FIG. 7 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system in single stagemode at the beginning of the heel boil-off and vapor recovery phase.

FIG. 8 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system in single stagemode during the vapor recovery phase.

FIG. 9 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system at a point when thesystem reaches a predetermined pressure during vapor recovery and may bestopped and changed to two stage mode where the cylinders are piped inseries.

FIG. 10 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system during deepevacuation vapor recovery and in two stage mode where the cylinders arepiped in series.

FIG. 11 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system at the completionof the deep evacuation vapor recovery phase at which point fluid flowhas stopped.

FIG. 12 is a schematic drawing illustrating an example implementation ofa liquefied gas unloading and deep evacuation system used on a singlerail car with a low maximum transfer rate.

FIG. 13 is an example of a control system diagram showing how the PLCcommunicates with the components of the liquefied gas unloading and deepevacuation system.

FIG. 14 is a block diagram showing an example of a control loop betweenthe PLC, the variable frequency drive, and the compressor motor.

FIG. 15 is a graph showing an example of the relationship between thefluid flow rate, the compression ratio, and the motor horsepower.

FIG. 16 is a graph showing an example of the differences in fluid flowrate at hotter and cooler temperatures and how that affects the motorhorsepower over time.

FIG. 17 is a graph showing an example of controlling peak power requiredby reducing the compressor speed.

DETAILED DESCRIPTION

The claimed subject matter is now described with reference to thedrawings, wherein like reference numerals are generally used to refer tolike elements throughout. In the following description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the claimed subject matter. It may beevident, however, that the claimed subject matter may be practicedwithout these specific details. In other instances, structures anddevices are shown in block diagram form in order to facilitatedescribing the claimed subject matter.

An example liquefied gas unloading and deep evacuation system 100 isshown and described. Liquid or liquefied gas may be any liquid gases,including without limitation, propane, propylene, ammonia, refrigerants,butane, or other liquefied gas. The implementation of a two stagecompressor, capable of both single stage and two stage operation, intothe offloading process of liquefied gases such as propane, propylene,ammonia, and refrigerants is advantageous. Switching the two stagecompressor from single stage operation to two stage operation throughthe quick and simple adjustment of control valves will allow thecustomer to benefit from the deep tank evacuation capabilities of a twostage compressor. Deep tank evacuation capabilities means the customerwill be able to recover additional liquid or product from the rail ortruck supply tank. A two stage compressor equipped with a variablefrequency drive (VFD) and a programmable logic controller (PLC) basedcontrol system has further advantages. The PLC is capable of maximizingthe operational speed of the compressor based on the temperature andpressures of the system. By maximizing the speed of the compressor, andtherefore maximizing the fluid transfer rate, within the limits of thepower rating of the compressor and the limits of the excess flow valvesfitted to the supply tanks, the offloading of the fluid is faster andmore efficient.

Liquid transfer and vapor recovery are shown in FIGS. 1 and 2. Withreference to FIG. 1, a diagram of an example implementation of aliquefied gas unloading and deep evacuation system 100 during liquidtransfer operation is shown. Before the liquid transfer process begins,pressure is equalized between both a supply tank 102 and a storage tank104. The supply tank 102 may be a rail car, truck transport tank, othertransport tank, or another stationary tank. When a two stage compressor106 is started it pulls vapor from a top 108 of the storage tank 104through transfer piping 110. The two stage compressor 106 may be a twostage reciprocating gas compressor. The two stage compressor 106compresses vapor and increases the vapor pressure as it is pushed into atop 116 of the supply tank 102. As differential pressure between thesupply tank 102 and the storage tank 104 increases, liquefied gas beginsto flow from the supply tank 102 to the storage tank 104 throughtransfer piping 110. A four way valve 114 having a first position 140enables the liquid transfer process to occur. The liquid transferprocess is now complete, but liquid heel remains.

With reference to FIG. 2, the compressor 106 is turned off to begin thevapor recovery process. The four way valve 114 may be reversed anddisposed in a second position 142 and other valves are opened or closedto facilitate the vapor recovery. The two stage compressor 106 isstarted and begins to draw the vapor from the supply tank 102. Theliquid heel, which may refer to the remaining liquid in the supply tank102 boils off during the recovery process. The liquid heel may accountfor about 0.5% of the total supply tank 102 volume. The two stagecompressor 106 compresses the vapor and increases the vapor pressure asit is pushed into the bottom 112 of the storage tank 104. The vaporrecovery process is complete when a desired evacuation pressure isreached.

FIGS. 3A-11 illustrate the liquefied gas unloading and deep evacuationsystem 100 in further detail. In one example implementation, theliquefied gas unloading and deep evacuation system 100 components may bedisposed on a skid 101. The liquefied gas unloading and deep evacuationsystem 100 may be powered by power means 126, such as electric motor128, combustion engine, hydraulic motor, or other means. The two stagecompressor 106 may comprise a first cylinder 120 and a second cylinder122. In one implementation, the first cylinder 120 may be larger thanthe second cylinder 122. In another implementation, the first cylinder120 may be about six inches in diameter, and the second cylinder 122 maybe about 3.25 inches in diameter. Piping outside of a compressor housing124 may enable the two stage compressor 106 to operate in a single stagemode and a two stage mode. The two stage compressor 106 may beselectably changeable between the single stage mode and the two stagemode at a predetermined pressure.

A liquid trap 130 may be fluidly coupled with the two stage compressor106 and the four way valve 114. The liquid trap 130 may be fluidlycoupled to a suction side of the two stage compressor 106. The liquidtrap 130 may trap liquid before it enters the two stage compressor 106.Even though the two stage compressor 106 may be fluidly coupled to vaporlines, small amounts of liquid may be present due to temperature changesand, thus, causing condensation to be in the vapor lines. At othertimes, incorrectly positioned valves may allow liquid to enter the vaporlines.

A two way valve 132 may selectably and fluidly couple the liquid trap130 with the two stage compressor 106. The two way valve may fluidlycouple the liquid trap 130 with the second cylinder 122 of the two stagecompressor 106. In another implementation, the two way valve 132 may beopen during the single stage mode of the two stage compressor 106, asshown in FIGS. 4-8. During the single stage mode and the two way valve132 in an open position, the first cylinder 120 and the second cylinder122 are piped in parallel such that vapor leaving the liquid trap 130 issplit into two lines, where one line enters the first cylinder 120 andthe second line passes through the open two way valve 132 and enters thetwo stage compressor 106 through the second cylinder 122. Operating thetwo stage compressor 106 with the two way valve 132 in the open positionenables for faster speeds and higher efficiency for liquid transfer fromthe supply tank 102 to the storage tank 104. The two way valve mayremain in the open position during the liquid transfer stage and theinitial vapor recovery stage. In a nonlimiting implementation, the twoway valve 132 may remain open until the predetermined pressure reachesabout 50 psi. In another implementation, the two way valve 132 mayremain open until the predetermined pressure reaches about 40 psi. Inyet another implementation, the two way valve 132 may remain open untilthe predetermined pressure reaches about 30 psi, 20 psi or 0 psi.

When the supply tank 102 pressure (or system suction pressure) reachesthe predetermined pressure to complete the initial vapor recovery stage,the two way valve 132 may be selectably changed to a closed position. Inthe closed position, the first cylinder 120 and the second cylinder 122are piped in series through a three way valve 134. FIG. 9 is an exampleimplementation of a pressure switch PS1 operably connected to the liquidtrap 130 and sensing the predetermined pressure. As shown in FIG. 9,when the predetermine pressure is for example, about 30 psi, thepressure switch PS1 signals the system 100 to switch the process switchto the deep evacuation stage. FIGS. 10 and 11 show the two way valve inthe closed position during the deep evacuation stage of the vaporrecovery process.

The three way valve 134 is shown in FIGS. 3-11. The three way valve 134may fluidly couple the two stage compressor 106 with the four way valve114. The three way valve 134 may be selectably changeable between afirst passageway 136 and a second passageway 138. The first passageway136 may fluidly couple the first cylinder 120 with the four way valve114. The second passageway 138 may fluidly couple the first cylinder 120with the second cylinder 122 of the two stage compressor 106. The firstpassageway 136 of the three way valve 134 may be open during the singlestage mode and may be closed during the second stage mode. The secondpassageway 138 of the three way valve 134 may be closed during thesingle stage mode and may be open during two stage mode.

In another implementation, the first passageway 136 and the secondpassageway 138 may be defined by two, two way valves 137 a, 137 b ratherthan the three way valve as shown in FIG. 4A.

The liquid transfer mode, the heel boil-off and vapor recovery stagesare shown in FIGS. 4-11. During the liquid transfer mode and heelboil-off and initial vapor recovery stages, the first passageway way 136may be open and the second passageway 138 may be closed. Vapor travelsfrom the liquid trap 130, through the first cylinder 120 and the secondcylinder 122 in parallel, and then passes through the first passageway136 of the three way valve 134. After passing through the three wayvalve 136, the vapor, under increased pressure from passing through thefirst cylinder 120, then travels to the four way valve 114 and eitherreturns to the supply tank 102 or the storage tank depending upon theposition of the four way valve 114.

Turning to FIG. 9 an example implementation of a pressure switch PS1operably connected to the liquid trap 130 and sensing the predeterminedpressure. As shown in FIG. 9, when the predetermine pressure is forexample, about 30 psi, the pressure switch PS1 signals the system 100 toswitch the process to the deep evacuation stage. The first passageway136 of the three way valve 134 closes and the second passageway 138opens. FIGS. 10 and 11 show the passageway 136 in the closed positionduring the deep evacuation stage of the vapor recovery process. Duringthe deep evacuation stage, vapor travels from the liquid trap 130,through the first cylinder 120, then the second passageway 138 of thethree way valve 134, and then the second cylinder 122. During thisstage, the first cylinder 120 and the second cylinder may be in series.Vapor under increased pressure exiting the second cylinder 122 thenpasses through the four way valve and returns to the storage tank 104.

The four way valve 114 is shown in FIGS. 1-11. The four way valve 114may comprise a first position 140 and second position 142. The four wayvalve 114 may have two, L-shaped passageways, and when rotated to thefirst position 140 or the second position 142, four passageways couplingvarious components of the system may be defined as further describedbelow. The first position 140 may comprise a first passageway 144fluidly coupling the three way valve 134 and the second cylinder 122with the supply tank 102. The first position 140 may also comprise asecond passageway 146 fluidly coupling the storage tank 104 with theliquid trap 130 during liquid transfer. The second position 142 maycomprise a third passageway 148 fluidly coupling the supply tank 102with the liquid trap 130 and a fourth passageway 150 that may fluidlycouple the three way valve 134 and the second cylinder 122 of the twostage compressor 106 with the storage tank 104 during vapor recovery.When the system 100 switches to the compressor 106 to the two stagemode, the four way valve remains in the second position 142 such thatthe third passageway 148 may fluidly couple the supply tank 102 with theliquid trap 130 and the fourth passageway 150 may fluidly couple thesecond cylinder 122 of the two stage compressor 106 with the storagetank 104.

FIG. 4A illustrates another implementation of the liquefied gasunloading and deep evacuation system 100, the four way valve 114 may bereplaced with four, two way valves 115 a, 115 b, 115 c, 115 d. One twoway valve 115 a may comprise the first passageway 144 and the second twoway valve 115 b may comprise the second passageway 146. The third twoway valve 115 c may comprise the third passageway 148 and the fourth twoway valve 115 d may comprise the fourth passageway 150. It should beunderstood that any combination of valves may be utilized to achieve thefour passageways fluidly coupling the components of the system 100. Thismay include the use of a three way valve and a two way valve or multipletwo way valves as previously described.

With references to FIGS. 1-12, an example of the liquefied gas unloadingand deep evacuation system 100 is shown and described in more detail.The supply tank 102, when in the form of a rail car, may have a dip tube152 and other piping for liquid transfer and vapor recovery. To fullyevacuate vapor and transfer as much liquid as possible, the process goesthrough a liquid transfer mode and a vapor recovery mode. The vaporrecovery mode may comprise a liquid heel boil-off stage, a vaporrecovery stage and a deep vapor evacuation stage.

FIGS. 5 and 6 show an example implementation of the liquid transfermode, which may also include an initial vapor recovery mode. Aspreviously described, when the two stage compressor 106 is started andit pulls vapor from the top 108 of the storage tank 104 through transferpiping 110. The two stage compressor 106 compresses vapor and increasesthe vapor pressure as it is pushed into the top 116 of the supply tank102. As differential pressure between the supply tank 102 and thestorage tank 104 increases, liquid begins to flow from the supply tank102 to the storage tank 104 through transfer piping 120. In this liquidtransfer mode, suction pulls vapor from the storage tank 104, throughthe second passageway 146 of the four way valve 114 and enters liquidtrap 130. The vapor may then enter the compressor 106. The two way valve132 is open and the three way valve 134 is positioned such that vaporfrom the liquid trap passes through the first cylinder 120 and thesecond cylinder 122 in parallel. This is accomplished through the twoway valve 132 being open and the first passageway 136 of the three wayvalve being open to the four way valve 114. Pressurized vapor exitingthe second cylinder 122 may combine with pressurized vapor exiting thefirst cylinder 120 after passing through the first passageway 136 of thethree way valve, and pass through the first passageway 144 of the fourway valve 114 and enters the supply tank 102. As pressure increaseinside the supply tank 102, the liquid is forced into the storage tank104 through piping 110.

With reference to FIG. 7, the vapor recovery mode is shown, and oneexample of the heel boil-off stage and the vapor recovery stage. Valvepositions may change. As shown in FIG. 7, the two way valve 132 remainsopen as does the first passageway 136 of the three way valve 134. Thecompressor 106 may maintain the single stage mode with the firstcylinder 120 and the second cylinder 122 being piped in parallel. Duringthis stage the four way valve 114 changes to the second position 142. Inone example implementation, movement from the first position 140 to thesecond position 142 of the four way valve 114 may be a ninety degreeturn of a handle of the four way valve. In the second position 142 ofthe four way valve 114, suction moves vapor from the supply tank 102through the third passageway 148 to the liquid trap 130. Vapor passesthrough the liquid trap 130 and enters the compressor 106. The vaporpasses through the compressor 106 as described. During the heel boil-offstage and the vapor recovery stage, pressurized vapor exiting the secondcylinder 122 and the first passageway 136 of the three way valve 134,the pressurized vapor travels through the fourth passageway 150 of thefour way valve 114 and enters the storage tank 104. This vapor recoverystage continues until the pressure switch PS1 detects a predeterminedpressure.

In one nonlimiting example, the predetermined pressure may occur at 50psi before the system enters the deep evacuation stage. In anotherimplementation, the predetermined pressure may be 40 psi before enteringthe deep evacuation stage. In yet another implementation, and asillustrated in FIG. 8, the vapor recovery stage may not occur until thepressure switch PS1 detects a pressure of 30 psi or even 20 psi.

With reference to FIG. 9, the pressure switch PS1 may detect apredetermined pressure, such as 30 psi. At such time, valve positionsmay again change as shown in FIGS. 10 and 11. Once the predeterminedpressure is reached, the deep evacuation stage may begin until thepressure switch PS1 detects a pressure of about 0 psi in the supply tank102. In the deep evacuation stage, the compressor 106 enters two stagemode. The two way valve 132 changes to the closed position. The firstpassageway 136 of the three way valve 134 closes, and the secondpassageway 138 of the three way valve 134 opens to fluidly couple thefirst cylinder 120 with the second cylinder 122 of the compressor 106.The four way valve 114 maintains the second position 142, which maycomprise the third passageway 148 and the fourth passageway 150 aspreviously described. During the deep evacuation stage, the vapor may besuctioned out. The vapor travels through the third passageway 148 of thefour way valve 114 into the liquid trap 130. The vapor leaves the liquidtrap and enters the first cylinder 120 where it is compressed. Thehigher pressurized vapor discharged from the first cylinder 120 travelsthrough the second passageway 138 of the three way valve 134 and entersthe second cylinder 122 of the compressor. The vapor is furthercompressed and discharged from the compressor 106. The compressed vaporenters the fourth passageway 150 of the four way valve 114 and goes tothe storage tank 104. The deep evacuation process may be consideredcompleted when the supply tank reaches about 0 psi or close to 0 psi,which may be any pressure less than 10 psi. At such time, the electricmotor 128 is turned off and the deep evacuation stage is completed.

The aforementioned liquid transfer and deep evacuation system describedabove may be utilized with two or more supply tanks 102. Each supplytank 102, such as a rail car, may have one or two excess flow valves 152that may not exceed 150 gallons/minute, or 300 gallons/minute per railcar. With two supply tanks 102, liquid transfer of 600 gallons/minutemay be achieved. In some implementations, the supply tank 102 may be asingle rail car. In an example implementation utilizing only one supplytank, the compressor 106 may operate in two stage mode for slower liquidtransfer, as shown in FIG. 12. This implementation may require a slowercompressor speed to remain at or slightly below 300 gallons/minutedepending on ambient temperature. The compressor 106 may operate atabout 400-825 RPM (depending on the ambient temperature) and provideabout a 300 gallons/minute liquid transfer rate year-round for unloadinga single rail car.

With references to FIGS. 13-17, in another implementation of theliquefied gas unloading and deep evacuation system 100, the two stagecompressor 106 may comprise variable speed. The system 100 may use avariable frequency drive (VFD) 154 to adjust and maximize the speed (andtherefore the capacity) of the compressor 106 during the liquid transferphase, vapor recovery, and the deep evacuation phase. In oneimplementation, the speed may vary due to ambient temperature as furtherexplained below.

The system 100 may use the variable frequency drive (VFD) 154 to adjustand maximize the speed (and therefore the capacity) of the compressor106 within the limits of the compressor's power rating and within theliquid flow limit as determined by the excess flow valves 118 operablyconnected to the supply tank(s) 102 being emptied. Supply tanks 102 andstorage tanks 104 used may be fitted with excess flow valves 118, whichallow a maximum flow rate at which liquid can be removed from a tank. Ifthe maximum flow rate is exceeded, the excess flow valve 118 closes,which stops or slows the liquid transfer process. For example, a typicalpropane supply tank 102, such as a rail car, may be fitted with twoexcess flow valves 118, each rated for 150 gallons per minute (GPM) fora total maximum liquid withdraw rate of 300 GPM. The compressor 106unloading two such supply tanks 102, such as rail cars, simultaneouslyshould not exceed a total liquid transfer rate of 600 GPM. The vaporpressure of the liquid, such as propane, is higher in the summer than inthe winter (as experienced in the northern and western hemispheres, forexample). Using a fixed compressor speed, this results in asignificantly higher liquid transfer rate in the summer than in thewinter. For example, a fixed speed compressor should be selected to notexceed a transfer rate of 600 GPM in the summer. This fixed speed mayresult in a transfer rate below 400 GPM in the winter due to the lowervapor pressure of the liquid, such as propane. The liquefied gasunloading and deep evacuation system 100 can vary the compressor speed(run faster in the winter) to maximize the liquid transfer rate in allseasons. The maximum benefit will be reduced unloading times during thewinter months, which may coincide with the busiest time of year in thepropane industry.

The new system 100 can also vary the compressor speed during the vaporrecovery process. At the beginning of the evacuation process, the powerrequired by the compressor 106 may be relatively low due to the lowcompression ratio. Toward the end of the evacuation process, the powerrequired by the compressor is also relatively low due to the lower massflow rate. However, in the middle of the evacuation process a power peakoccurs. In a fixed speed compressor, the compressor speed must beselected to not exceed the peak power rating of the compressor or motor.The new system 100 can vary the compressor speed to operate at higherspeeds at the beginning and end of the evacuation process, and slower inthe middle to avoid exceeding the power rating of the compressor ormotor. The effect of this is to reduce the time required for theevacuation process.

With reference to FIGS. 13 and 14, a control system 156 is shown. Aprogrammable logic controller (PLC) 158 enables an operator to monitoroperating conditions and environmental conditions to optimize theoperation of the compressor 106. Environmental and operating conditionsare sent to the PLC. The environmental conditions may include withoutlimitation, ambient temperature. Operating conditions may includewithout limitation, an operator interface, suction pressure, dischargepressure, low oil pressure, high discharge temperatures, a high liquidalarm and/or a high liquid shutdown. In one implementation, controltransmitters may be used to transmit data relating to ambienttemperature, suction pressure and discharge pressure. In anotherimplementation, the control system 156 may utilize signals fromtransmitters to detect suction pressure, discharge pressure, low oilpressure, high discharge temperatures, high liquid alarm and high liquidshutdown as parameters for safety shutdowns. As shown in FIG. 14, acontrol loop may be defined by the PLC 158, the VFD 154, and the motor128. The PLC 158 may provide a frequency signal (Hz) to the VFD 154. Thefrequency signal to the VFD 154 may correlate to a motor speed inrevolutions per minute. The VFD 154 may provide feedback data in theform of motor amperage information. Depending upon the feedback motoramperage information, the PLC 158 may then adjust the frequency signalto maintain a maximum motor amperage. This includes the optimumcompressor speed (capacity) for the environmental conditions. The system100 may transfer liquid faster, evacuate the tank faster, and recovermore product from each tank.

FIGS. 15-17 illustrate nonlimiting implementations of utilizing the VFD.Turning to FIG. 15, suction pressure (mass flow rate) is shown to beinversely proportional to the compression ratio over time. At thebeginning of the heel boil-off/vapor recovery stage, the mass flow rateis high, there is a low compression ratio, and the system uses lowhorsepower. During the middle of the cycle during the vapor recoverystage, both the mass flow rate and the compression ratio is medium, butthe system experiences high horsepower. During the deep evacuationstage, there is a high compression ratio, a low flow rate and lowhorsepower. As shown in FIG. 15, horsepower peaks during mid-cycle.Turning to FIG. 16, the same pattern is shown with the addition ofshowing warmer and cooler ambient temperature. In cooler weather, vaporpressure (suction pressure) is lower. In cooler temperatures, there is alower horsepower peak. To ensure that the horsepower does not exceed thelimit of the compressor, and simultaneously, and to compensate for thehorsepower peak during mid-cycle, the liquefied gas unloading and deepevacuation system 100 lowers the speed of the compressor only whennecessary to keep the horsepower within the limits of the compressor. Byutilizing the control system 156 with the control loop between the PLC158 and the VFD 154, the electric motor 128, and thus the speed of thecompressor 106, adjusts during mid-cycle so that compressor speed isalways operating at the highest rpm possible during the various stageswithout exceeding the horsepower limit. In one implementation the liquidheel boil-off stage may comprise a speed S1, the second vapor recoverystage may comprise a speed S2, and the deep vapor evacuation stage maycomprise a speed S3, wherein S2 is less than S1 and S3 as shown in FIG.17. This variable speed drive technique can be applied to eithertwo-stage compressors or a traditional single-stage compressor withidentical benefits.

The word “exemplary” is used herein to mean serving as an example,instance or illustration. Any aspect or design described herein as“exemplary” is not necessarily to be construed as advantageous overother aspects or designs. Rather, use of the word exemplary is intendedto present concepts in a concrete fashion. As used in this application,the term “or” is intended to mean an inclusive “or” rather than anexclusive “or.” That is, unless specified otherwise, or clear fromcontext, “X employs A or B” is intended to mean any of the naturalinclusive permutations. That is, if X employs A; X employs B; or Xemploys both A and B, then “X employs A or B” is satisfied under any ofthe foregoing instances. Further, at least one of A and B and/or thelike generally means A or B or both A and B. In addition, the articles“a” and “an” as used in this application and the appended claims maygenerally be construed to mean “one or more” unless specified otherwiseor clear from context to be directed to a singular form.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. Of course, those skilled inthe art will recognize many modifications may be made to thisconfiguration without departing from the scope or spirit of the claimedsubject matter.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure includes all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of thedisclosure.

In addition, while a particular feature of the disclosure may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“includes,” “having,” “has,” “with,” or variants thereof are used ineither the detailed description or the claims, such terms are intendedto be inclusive in a manner similar to the term “comprising.”

The implementations have been described, hereinabove. It will beapparent to those skilled in the art that the above methods andapparatuses may incorporate changes and modifications without departingfrom the general scope of this invention. It is intended to include allsuch modifications and alterations in so far as they come within thescope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A liquefied gas unloading and deep evacuationsystem, comprising: a variable speed two stage compressor comprising asingle stage mode and a two stage mode; the two stage compressorcomprising a first cylinder and a second cylinder; a liquid trap fluidlycoupled with the variable speed two stage compressor; a pressure switchoperably connected to a discharge of the compressor and configured tosense a differential pressure; a control system, comprising a controlloop defined by a programmable logic controller, a variable frequencydrive, and a motor; the programmable logic controller operablycommunicable with the two-stage compressor, the programmable logiccontroller configured to operably control the variable speed of thetwo-stage compressor and operably transitioning the two stage compressorbetween the single stage mode and the two stage mode at a predetermineddifferential pressure based on operating conditions; the variablefrequency drive operably communicable with the programmable logiccontroller configured to provide feedback data to the programmable logiccontroller so the programmable logic controller can operably control thevariable speed of the two-stage compressor; a three way valve fluidlycoupling the two stage compressor with a four way valve, the three wayvalve selectably changeable between a first passageway and a secondpassageway, the first passageway fluidly coupling the first cylinderwith the four way valve, and the second passageway fluidly coupling thefirst cylinder with the second cylinder of the two stage compressor; thefour way valve comprising a first position and second position the firstposition comprising a first passageway of the four way valve fluidlycoupling the three way valve and the second cylinder with a supply tankand a second passageway fluidly coupling a storage tank with the liquidtrap during the single stage mode; the second position comprising athird passageway fluidly coupling the supply tank with the liquid trapand a fourth passageway fluidly coupling the three way valve and thesecond cylinder of the two stage compressor with the storage tank duringthe single stage mode; the second position comprising the thirdpassageway fluidly coupling the supply tank with the liquid trap and thefourth passageway fluidly coupling the second cylinder of the two stagecompressor with the storage tank during the two stage mode; furthercomprising a liquid heel boil-off/vapor recovery stage comprising acompressor speed S1, a second vapor recovery stage comprising acompressor speed S2, and the deep vapor evacuation stage comprising acompressor speed S3, wherein the compressor speed at S2 is less than thecompressor speed at both S1 and S3; the deep vapor evacuation stagebeing completed when a supply tank reaches a pressure less than 10 psi;the deep vapor evacuation stage being in the two stage mode.
 2. Theliquefied gas unloading and deep evacuation system of claim 1, whereinthe predetermined pressure is about 50 psi.
 3. The liquefied gasunloading and deep evacuation system of claim 1, wherein thepredetermine pressure is about 30 psi.
 4. The liquefied gas unloadingand deep evacuation system of claim 1, wherein the supply tank is a railcar.
 5. The liquefied gas unloading and deep evacuation system of claim1, comprising a two way valve selectably and fluidly coupling the liquidtrap with the two stage compressor, the two way valve being open duringthe single stage mode and closed during the two stage mode.
 6. Theliquefied gas unloading and deep evacuation system of claim 5, whereinthe two way valve is fluidly coupled with the second cylinder of the twostage compressor.
 7. The liquefied gas unloading and deep evacuationsystem of claim 1, wherein the first passageway of the three way valveis open during the single stage mode and closed during the second stagemode, and the second passageway of the three way valve is closed duringthe single stage mode and open during second stage mode.
 8. Theliquefied gas unloading and deep evacuation system of claim 1, whereinthe first cylinder and the second cylinder are in parallel in the singlestage mode, the first cylinder and the second cylinder are in series inthe two stage mode.
 9. The liquefied gas unloading and deep evacuationsystem of claim 1, the two stage compressor comprising a first speed anda second speed, the first speed being greater than the second speed. 10.The liquefied gas unloading and deep evacuation system of claim 1,further comprising a programmable logic controller operably controllingthe variable speed of the two stage compressor.
 11. The liquefied gasunloading and deep evacuation system of claim 10, the programmable logiccontroller operably controlling the variable speed of the two stagecompressor from one or more of: ambient temperature; suction pressure;discharge pressure low oil pressure; high discharge temperature; highliquid alarm; and/or high liquid shutdown.
 12. The liquefied gasunloading and deep evacuation system of claim 1, wherein the liquefiedgas is one or more of propane, propylene, and butane.